The broad objective of this investigation is to describe how a variety of nonvolatile organic compounds penetrate lung membranes to enter the bloodstream after inhalation as aerosols. More specifically: (1) To measure the pulmonary absorption rates for various drugs, nutrients, dyes, and other environmental substances. (2) To elucidate the mechanisms of these absorption processes through studies of absorption kinetics. (3) To correlate rates and mechanisms of lung absorption with physicochemical properties of the compounds, so that the permeability characteristics of the membrane are defined, and we are able to predict rates of pulmonary absorption of other compounds from their simple physical and chemical properties. (4) To compare lung permeability in a variety of mammalian species so that the information will have broad application and a high probability of extending to man. (5) To study lung permeability in the neonatal animal as well as in the old adult animal and to look for age-related changes in various species.